How full was the tank when you found the icicle? If the tank was only half full it takes things like the vent pipe out of the equation.
You're going to have to fill the tank fuller to test this. I would fill the tank to the top of the tank by looking down into the toilet while filling. Use low pressure from a hose so you don't get any splash back in the RV or in your face.
Let it sit for an hour.
If no leaks the problem is either the vent pipe seal or the toiled to tank gasket. Personally if I couldn't find a leak with the first test I would just pull the toilet and replace the gasket. It's a whole two nuts and unscrew the water line and the toilet is out. Remove the old foam gasket and put a new RV toilet gasket in place and reinstall the toilet.
If you really want to explore further after replacing the gasket then fill the tank until the water is up into the toilet bowl and see if you get any leaks.
Could the leak have been from an overflowed gray tank?
Also I would crawl under the trailer and look up around where the icicle was. Are you sure the icicle was waste and not stained water leaking down the walls from a roof leak?